The present, invention refers to an article plated with boron carbide in a nickel-phosphorus matrix, to the process of preparation of said article by electrodeposition of inert particles in a nickel-phosphorus matrix, and to an electrolytic, bath for obtaining said article.
In the sector of the production of articles that are subjected to severe wear and, more in particular, in the sector for producing cylinders for the production of corrugated cardboard, the role of the plating applied on the article, and in particular of the plating applied on the cylinder, is of fundamental importance. In fact, it is precisely the plating that must be able to withstand severe wear. There are two types of plating currently used in the said sector: classic chrome plating, with thicknesses of the order of 100 xcexcm, deposited electrolytically, and tungsten-carbide plating.
Albeit having an acceptable cost, classic chrome plating presents a number of drawbacks. In the first place, it is not possible to obtain uniformity of plating; namely, plating is deficient in the groove and abundant on the sharp point on account of the point effect. As a result, the thickness in the groove may be 70% less than the thickness on the sharp point, even though an appreciable hardness is maintained (approximately 850 Hv).
The second type of plating used in the sector of plating of articles that undergo severe wear, such as cylinders for the production of corrugated cardboard, is tungsten-carbide plating. Albeit having good characteristics of hardness (approximately 1300 Hv), good uniformity, and good adhesion, this type of plating presents very high costs which render this type of solution far from economically convenient and not usable for a large number of industrial applications.
Now according to the present invention, a plated article has been found, as well as a method for its preparation, and an electrolytic bath for application of said plating which enable the drawbacks of the prior art to be overcome.
In particular, a subject of the present invention is an electrolytic bath for the deposition of a plating of boron carbide in a nickel-phosphorus matrix which comprises two or more nickel salts, at least one complexing agent, and at least one phosphorus salt, characterized in that it contains an anti-tensioning agent and boron carbide in the form of powder.
A further subject of the present invention is an article plated with boron carbide in a nickel-phosphorus matrix.
Yet another subject of the present invention is a process for the production of a plating with boron carbide in a nickel-phosphorus matrix, characterized in that it comprises the following steps:
a) preparation of an electrolytic bath comprising two or more nickel salts, at least one complexing agent, at least one phosphorus salt, and, in addition, an anti-tensioning agent and boron carbide in the form of a powder;
b) electroplating of an article in said electrolytic bath at a temperature ranging from 40xc2x0 C. to 70xc2x0 C., with a current density ranging from 1 to 10 A/dm2; and
c) heat treatment of the product thus plated.
In particular, the electrolytic bath is used for the plating treatment of articles to obtain plated articles having the desired quality.
The main advantage of the plating process according to the present invention consists in that easy management of the entire process is combined with a low cost, and hence excellent possibility of application in the industrial field, and at the same time a plated product is obtained that is provided with high resistance to wear. In addition, a further advantage of the plating obtained using the process according to the present invention lies in its excellent adhesion to the surface of the article to be plated.
The electrolytic bath according to the present invention may comprise, as nickel salts, nickel sulphate and nickel chloride.
Preferably, the electrolytic bath contains, as phosphorus salt, salts of phosphorous acid and of hypophosphorous acid.
The complexing agents are chosen from among citric acid, lactic acid, malic acid, malonic acid, succinic acid, glycolic acid, and short-chain carboxylic acids.
Preferably, the complexing agent is lactic acid.
The anti-tensioning agents are chosen between saccharin and compounds of diethylcarbamic acid, preferably saccharin.
Preferably, the electrolytic bath according to the present invention contains from 0.01 mol. to 3 mol. of nickel sulphate, from 0.003 mol. to 2 mol. of nickel chloride, from 0.006 mol. to 1.8 mol. of the salt of phosphorous acid, from 0.05 mol. to 2 mol. of lactic acid, from 0.2 g/l to 30 g/l of saccharin, and from 0.2 g/l to 30 g/l of boron carbide.
More preferably still, the electrolytic bath according to the present invention contains from 0.1 mol. to 2 mol. of nickel sulphate, from 0.01 mol. to 1.5 mol. of nickel chloride, from 0.06 mol. to 1.0 mol. of the salt of phosphorous acid, from 0.1 mol. to 1 mol. of lactic acid, from 3 g/l to 10 g/l of saccharin, and from 5 g/l to 15 g/l of boron carbide.
In particular, the particles of boron carbide have a grain size of between 3 and 6 xcexcm.
The said bath can be used with a current density of from 1 to 10 A/dm2, at a temperature ranging between 40xc2x0 C. and 70xc2x0 C., under stirring, whilst the pH value of the electrolytic bath may range from 0.4 to 10. The temperature of the bath and the current density for the plating process have been chosen in the range referred to above for the reasons explained in what follows. At temperatures lower than 40xc2x0 C., the current density would not be sufficient, and there would be a low efficiency of electrodeposition. At temperatures higher than 70xc2x0 C. the disadvantage of the high evaporation of the bath would exceed the advantage due to an increase in the efficiency of electrodeposition.
Step b) of the plating process according to the present invention is preferably performed at a temperature of 60xc2x0 C., with a current density of 2 A/dm2, under stirring.
Step c) is performed at a temperature within the 250xc2x0 C.-400xc2x0 C. range, and preferably at a temperature of 340xc2x0 C., for 12 hours.
The material for the cathode is the material to be plated, whilst the anode can be chosen from among anodes made of electrolytic nickel.
In particular, the plated article obtained with the process according to the present invention is a cylinder for the production of corrugated cardboard.
The aforesaid plating can, in any way, be used for any type of article which, in any sector of application, requires plating that is resistant to wear, such as aluminium articles.
The plated article according to the present invention may undergo further treatments, such as a polishing treatment using diamond paste.
In the particular case of an article having a poorly receptive surface, such as a nitrided surface, in order to obtain better adhesion of the plating with boron carbide in a nickel-phosphorus matrix, the plating process according to the invention may be advantageously preceded by the following pre-treatment steps:
1) sanding;
2) chemical degreasing;
3) washing;
4) neutralization in 10% sulphuric acid;
5) washing;
6) deposition of chemical nickel from an alkaline solution at 35-43xc2x0 C., the said solution being known under the commercial name ENPLATE AL 100; and
7) further washing.
In particular, sanding may be performed using a machine at a pressure of 7 bar and employing corundum with grain size 150; chemical degreasing may be carried out using ultrasound (6 W per litre) at a temperature of 75xc2x0 C.; the first washing may be carried out in purified water circulated on activated carbon, whilst the washing referred to in point 4) is carried out in purified water circulated on dolomite; deposition of chemical nickel is carried out in cold conditions, keeping the pH value above 9.6 to prevent formation of clouding of the solution and of dark deposits; the final washing consists of a first static washing in deionized water and of a second washing in demineralized water circulated on resins. The plating process according to the invention is then applied on the product thus obtained.
The characteristics and advantages of the product according to the present invention will emerge more clearly from the ensuing detailed description, which is given purely to provide non-limiting examples.